High-performance GaN-based Light-emitting Diodes Research Articles

Study of a GaN-Based Light-Emitting Diode with a Specific Hybrid Structure

A specific hybrid structure, including 45° sidewalls, a shallower (0.25 μm in depth) microhole array, and a 3-D like backside reflector, is used to fabricate GaN-based light-emitting diodes (LEDs). The 3-D like backside reflector is formed by the deposition of 100 nm-diameter SiO2 nanospheres (NSs) and an Al layer on the sapphire substrate. Based on the use of 45° sidewall and microhole array, the reduced total internal reflection (TIR) and Fresnel reflection and the increased scattering probability and the opportunity of photons to find escape cones can be expected. In addition, the use of 3-D like backside reflector can effectively reflect scatter, and redirect the downward photons emitted from multiple quantum well (MQW) region into arbitrary directions. Thus, the light extraction performance can be substantially improved. In experiment, under an injection current of 400 mA, the studied Device D with this hybrid structure shows enhancements of 47.4%, 47.7%, 33.1%, and 47.8% in light output power (LOP), external quantum efficiency (EQE), luminous flux, and wall-plug efficiency (WPE), respectively, as compared to a conventional LED. Moreover, the related electrical properties are not degraded for the studied Device D. Therefore, this hybrid structure shows a promise for high-performance GaN-based LED applications.

Influences of Microhole Depth and SiO2 Nanoparticle/Microsphere Passivation Layer on the Performance of GaN-Based Light-Emitting Diodes

A hybrid structure, including 45° sidewalls, a microhole array, and a thin SiO2 nanoparticle (NP)/microsphere (MS) passivation layer, is used to produce GaN-based light-emitting diodes (LEDs). The influences of the microhole depth and SiO2 NP/MS passivation layer on the LED performance are studied. A shallower depth of a microhole array shows better optical properties due to the complete preservation of the GaN/InGaN multiple quantum well (MQW) region. In addition, the use of a thin SiO2 NP/MS passivation layer gives a remarkably reduced reverse-biased leakage current and improved optical performance. Experimentally, under an injection current density of 110 A/cm2, the studied device, with a proper hybrid structure, shows enhancements of 25.9%, 29.2%, and 29.4% in light output power (LOP), external quantum efficiency (EQE), and wall-plug efficiency, respectively, as compared to a LED device with a deeper depth of microhole array. These improvements are mainly caused by the reduced total internal reflection (TIR) and the Fresnel reflection, which increase scattering probability and the opportunity to find photon escape cones. Hence, the studied hybrid structure in this work is a promising route to fabricate high-performance GaN-based LEDs.

Performance Improvement of GaN-Based Light-Emitting Diodes With a Microhole Array, 45° Sidewalls, and a SiO2 Nanoparticle/Microsphere Passivation Layer

The characteristics of GaN-based light-emitting diodes (LEDs) with a hybrid structure incorporating a microhole array, 45 sidewalls, and an appropriate SiO2 nanoparticle (NP)/microsphere (MSs) passivation layer are studied and reported. The use of a SiO2 NP/MSs passivation layer causes a remarkable reduction in reverse-biased leakage current. The employment of this hybrid structure leads to substantial enhancements in optical properties without any degradation in electrical performance. In addition, a lower content of SiO2 NP in the mixed SiO2 NP/MSs solution leads to enhanced optical behavior due to the improved transmittance. Experimentally, as compared with a conventional LED (Device A), the studied Device E shows 50.6%, 50.9%, 48.4%, and 49.9% enhancements in light output power, luminous flux, luminous efficacy, and wall-plug efficiency, respectively. These advantages are mainly attributed to the increased scattering probability and the opportunity to find photon escape cones as well as the reduced total internal reflection and Fresnel reflection effects. Therefore, the studied hybrid structure provides a promise for high-performance GaN-based LED applications.

Improvement of crystallinity and luminescence of GaN-based laser diode structure with suppressed curvature variation in active layers

The influence of curvature variation on the crystallinity and luminescence of GaN-based blue laser diode (LD) structure is comprehensively investigated during the growth of InGaN/GaN active layers. Compared with InGaN/GaN multiple quantum well (MQWs) grown by the conventional 2-temperature deposition, curvature variation is successfully suppressed and a much gentler deformation from wells to barriers is obtained by employing same-temperature deposition method (STDM). With less curvature fluctuation, the V-pit density of InGaN/GaN MQWs grown on sapphire is effectively decreased from 4.5 × 108/cm2 to 2.8 × 108/cm2 with average root-mean-square roughness of 0.55 nm, while threading dislocation density of the GaN-based LD structures grown on FS-GaN is also reduced from 5.9 × 106/cm2 to 1.6 × 106/cm2. Additionally, an ~ 7 nm redshift in photoluminescence emission for LD structure is achieved, accompanied by a ~ 6 times higher emission intensity. A more uniform distribution of emission wavelength along the radial direction is observed and the full width at half maximum is also narrowed, indicating that the STDM is advantageous to effectively eliminate the negative influence of curvature variation and contribute to fabricating high-performance GaN-based light-emitting diodes and LDs.

Enhanced Light Extraction of GaN-Based Light-Emitting Diodes With a Hybrid Structure Incorporating Microhole Arrays and Textured Sidewalls

GaN-based light-emitting diodes (LEDs) with a hybrid structure, including a microhole array and textured sidewalls (45° angle and convex patterns), are fabricated and studied. The use of this hybrid structure, formed using an inductively coupled plasma etching approach, causes reduced total internal reflection and increased scattering probability of photons at the GaN/air interface. Therefore, the opportunity for photons to find an escape cone and the related light extraction efficiency can be effectively increased. This designed hybrid structure provides substantial improvements in optical performance without any degradation in electrical properties. For instance, as compared with a conventional LED, the studied device with this hybrid structure (45° angle sidewalls) shows remarkable improvements of 20.9%, 24.3%, 20.5%, and 21.3% in light output power, luminous fluxes, external quantum efficiency, and wall-plug efficiency, respectively. In addition, higher intensities in the light emission mapping images and far-field patterns of the studied device are found. Therefore, the studied hybrid structure is promising for high-performance GaN-based LED applications.

Emission enhancement of light-emitting diode by localized surface plasmon induced by Ag/p-GaN double grating

In this paper, a new type of light-emitting diodes (LEDs) structure is designed to enhance the light emission efficiency of GaN-based LEDs. The structure mainly includes Ag grating, ITO layer and p-GaN grating. The principle of stimulating the localized surface plasmon to improve the luminous characteristics of the LED by using this structure is discussed. Based on the COMSOL software, the finite element method is used to simulate the LED structure. The normalized radiated powers, the normalized absorbed powers under different wavelength and geometric parameters, and the distribution of the electric field with the particular geometric parameters are obtained. The simulation results show that with a local ITO thickness of 32 nm, an etching depth of 29 nm, a grating period of 510 nm and a duty ratio of 0.5, the emission intensity of the designed GaN-based LED structure has increased by nearly 55 times than the ordinary LED providing a reliable foundation for the development of high-performance GaN-based LEDs.

Enhanced performance of GaN-based LEDs via electroplating of a patterned copper layer on the backside

InGaN/GaN multi-quantum well light-emitting diodes (LEDs) are conventionally grown on a sapphire substrate due to a lack of compatible substrates with a high compressive strain. This is a result of the relatively large lattice, and thermal expansion coefficient mismatches between GaN and sapphire. The compressive strain is considered to be a major obstacle to further improve next-generation high-performance GaN-based LEDs. In this paper, we have designed, electroplated, and tested an efficient substrate using a patterned copper (Cu) layer on the backside of sapphire to relax the compressive strain in a GaN epilayer. The patterned Cu layer has a significant function in that it supports the GaN/sapphire LEDs with an external tensile stress. The external tensile stress is capable of compensating for the compressive strain in the GaN/sapphire LEDs by controlling the curvature of the wafer bowing. This patterned Cu layer, when applied to the GaN/sapphire LEDs, suppresses the compressive strain by up to 0.28 GPa. The GaN-based LEDs on this innovative and effective sapphire/Cu substrate offer improved optical and electrical performance.

High-Performance GaN-Based LEDs With AZO/ITO Thin Films as Transparent Contact Layers

The Al-doped ZnO (AZO)/indium tin oxide (ITO) bilayer films were proposed as transparent contact layers (TCLs) for the fabrication of GaN-based light-emitting diodes (LEDs). In TCLs on the p-type GaN layer, the ITO film serves as the ohmic contact layer with the thickness of only 20 nm, whereas the 300–750nm AZO films act as the current spreading layer. At an optimal AZO thickness of 500 nm, GaN-based LEDs with AZO/ITO TCLs have a forward voltage of 3.36 V, an electroluminescence emission at 526 nm with highest brightness and a large light intensity of 386 mcd at 20 mA, which are almost the same as the commercial LEDs with a 300-nm ITO TCL. The 500-nm AZO/20-nm ITO bilayer films exhibit a high transparency above 90% in the visible region, but they display a low conductivity with resistivity $\sim 7 \times 10^{-3}~\Omega \cdot $ cm. The success of GaN-based LEDs with such a relatively high-resistivity electrode strongly demonstrates that the AZO/ITO film is a very effective and practical TCL on the p-type GaN layer. The indium-saving AZO/ITO TCLs may be actually a universal approach as p-type electrodes in high-performance GaN-based LEDs for commercially mass production with low cost.

Achieving High-Performance Blue GaN-Based Light-Emitting Diodes by Energy Band Modification on Al<italic>x</italic>In<italic>y</italic>Ga1–<italic>x</italic>–<italic>y</italic>N Electron Blocking Layer

We have achieved high-performance blue GaN-based light-emitting diodes (LEDs) by energy band modification on an Al x In y Ga1– x – y N electron blocking layer (EBL). It is demonstrated by simulation that the strategy using high In/Al ratio to decrease polarization charge density and to alleviate the negative effect from polarization electric fields is more favorable. Using the optimal In/Al ratio of 0.5, the LED with Al0.40In0.20Ga0.40N EBL shows a comprehensive performance improvement. With comparison to the LED with conventional Al0.15GaN0.85N EBL, such an LED owns a 270.15% improvement on the simulated luminous intensity and shows the smallest voltage of 3.34 V at 100 A/cm2. The subsequent experimental results provide the evidence to the superiority of Al x In y Ga1– x – y N1– x – y N EBL with a high In/Al ratio. Compared with the reference LED with Al0.15Ga0.85N EBL, the experimental light output power of the LED with Al0.35In0.18Ga0.47N EBL is 86.58 mW, achieving the best improvement by 31.38%. And the voltage of this LED at 14.4 A/cm2 (20 mA) is 3.35 V, which is much smaller than the voltage of 3.45 V for the reference LED. Evidently, the Al x In y Ga1– x – y N EBL with the In/Al ratio of 0.5 is very effective to promote the performance of LED, which is very promising to be applied in the further high-performance LED fabrication.

Effects of Nanoscale V-Shaped Pits on GaN-Based Light Emitting Diodes

This paper reviews the formation of nanoscale V-shaped pits on GaN-based light emitting diodes (LEDs) grown by the metal organic chemical vapor deposition (MOCVD) system and studies the effect of V-shaped pits on quantum efficiency. Since V-pits could provide potential barriers around threading dislocations to lessen non-radiative recombinations in such a high defect environment. In our study, multiple InGaN/GaN quantum well samples with different emission wavelengths of 380, 420, 460, and 500 nm were grown, each with different nanoscale V-shaped pits of three diameters for 150, 200, and 250 nm, respectively. It was found that the multiple quantum well (MQW) sample with larger V-pits had a lower pit density, but a relatively larger total V-pits defected area. The optimum diameter of V-pits showing the highest quantum efficiency from the MQW sample depended on the emission wavelength. MQW samples with wavelengths of 380 and 500 nm exhibited the best internal quantum efficiency (IQE) performance at the smallest V-pits area; however, the best performance for MQW samples with wavelength around 420 and 460 nm occurred when large V-pit areas were presented. Photoluminescence (PL) peak shifts and Raman shifts can provide a relationship between quantum-confined Stark effect (QCSE) and IQE, as well as a comparison between strain and IQE. The results obtained in this phenomenological study shall provide a useful guide line in making high-performance GaN-based LEDs with wide emission spectra.

Employing low-temperature barriers to achieve strain-relaxed and high-performance GaN-based LEDs.

The epitaxial structure design of low-temperature barriers has been adopted to promote strain relaxation in multiple quantum well (MQWs) and achieve high-efficient GaN-based light-emitting diodes (LEDs). With these barriers, the relaxation value of wells increases from 0 to 4.59%. The strain-relaxed mechanism of low-temperature barriers is also discussed. The LED chip with the barriers grown at the TMIn flow of 75 sccm and the growth temperature of 830 °C has an optimal strain relaxation value of 1.53% in wells, and exhibits the largest light output power of 63.83 mW at the injection current of 65 mA, which is higher than that of conventional LED (51.89 mW) by 23%. In-depth studies reveal that the optimal low-temperature barriers remarkably promote the strain relaxation in wells without forming large density of crystalline defects. This achievement of high-efficiency LEDs sheds light on the future solid-state lighting applications.

High-Performance Ultraviolet 385-nm GaN-Based LEDs With Embedded Nanoscale Air Voids Produced Through Atomic Layer Deposition and Al2O3Passivation

This letter developed the high-performance GaN-based light-emitting diodes (LEDs) for ultraviolet (UV) light emission at 385 nm with the light output power enhanced by up to 52% compared with that of the conventional UV-LEDs. The high-power-efficiency UV-LEDs include sidewall-passivated Al2O3 dielectric nanoscale air voids, which were created using the atomic layer deposition method and act as intermediate media for improving the internal quantum efficiency, reducing threading dislocation, and relaxing strain. The fabricated air void nanostructure also enhanced the light extraction efficiency by $\sim 32.7$ %, resulting in a high-power UV light-emitting device.

Super-aligned carbon nanotubes patterned sapphire substrate to improve quantum efficiency of InGaN/GaN light-emitting diodes.

In this paper, the high performance GaN-based light-emitting diodes (LEDs) on carbon-nanotube-patterned sapphire substrate (CNPSS) by metal-organic chemical vapor deposition (MOCVD) are demonstrated. By studying the mechanism of nucleation, we analyze the reasons of the crystal quality improvement induced by carbon nanotubes (CNTs) in different growth process. Combining with low temperatures photoluminescence (PL) measurements and two-dimensional (2D) finite difference time-domain (FDTD) simulation results, we conclude that the improvement of optical properties and electrical properties of CNPSS mainly originates from the improvement of the internal quantum efficiency (IQE) due to decreased dislocation density during nano-epitaxial growth on CNPSS. Additionally, in order to reduce the light absorption characteristics of CNTs, different time annealing under the oxygen environment is carried out to remove part of CNTs. Under 350 mA current injections, the light output power (LOP) of CNPSS-LED annealed 2 h and 10 h exhibit 11% and 6% enhancement, respectively, compared to that of the CNPSS-LED without annealing. Therefore, high temperature annealing can effectively remove parts of CNTs and further increase the LOP, while overlong annealing time has caused degradation of the quantum well resulting in the attenuation of optical power.

Effect of layers of carbon-nanotube-patterned substrate on GaN-based light-emitting diodes

In this paper, the high-performance GaN-based light-emitting diodes (LEDs) with coated carbon nanotubes (CNTs) on sapphire substrates, fabricated by metal–organic chemical vapor deposition (MOCVD), were demonstrated. The different layers of a CNT-patterned sapphire substrate (CNPSS) grown by an optimized growth process were discussed. Results of X-ray diffraction (XRD) showed the threading dislocations to be suppressed, thus the crystal quality of the GaN film was improved by introducing the carbon nanotube films. The LEDs with a CNPSS exhibited lower reverse-bias current and divergent angle, and larger enhancement of the light output power (LOP) compared with the conventional LEDs. With the increase in the number of layers of CNTs, the CNPSS-LED exhibited better crystal quality and photoelectric property, but more layers of CNTs also absorbed more light. There is a trade-off between the crystalline quality of the LED and light absorption.

Silver nanowires for transparent conductive electrode to GaN-based light-emitting diodes

Transparent, conductive, and uniform Ag nanowires (NWs) were introduced to improve the optical performance of GaN-based light-emitting diodes (LEDs) by a spin-coating technique. The Ag NWs acted as a current spreading layer, exhibiting high transmittance and low sheet resistance, and ultimately leading to high performance GaN-based LEDs with an ultra large size of 5 × 5 mm2. Compared to the transmittance of conventional LEDs without Ag NWs, the relative transmittance of LEDs with Ag NWs was approximately 90% of the overall wavelength region. However, the electroluminescence (EL) intensity of LED with Ag NWs was much higher than that of conventional LEDs without Ag NWs for injection current above 45 mA. In addition, the EL full width at half maximum of LEDs with Ag NWs was much lower than that of conventional LEDs without Ag NWs. Based on these results, we believe that the enhanced optical performance of ultra large LEDs was due to an increase in the current spreading effect.

High-performance light-emitting diodes using hierarchical m-plane GaN nano-prism light extractors

We demonstrated high-performance GaN-based light-emitting diodes (LEDs) with hierarchical m-plane nano-prism light extractors on the n-GaN mesa sidewalls by using a tetramethylammonium hydroxide (TMAH)-based crystallographic etching technique.

Performance enhancement by different sidewall structures for InGaN-based light-emitting diodes

We demonstrate high-performance GaN-based light-emitting diodes (LEDs) using different sidewall structure. The patterned pyramidal sidewall (PPS) structure in LED is fabricated by crystallographic etching process, which is applying hot phosphoric acid wet etching at the GaN/Al2O3 interface after normal front-side laser scribing. To obtain continuous, regular and bigger pyramidal sidewalls we applied higher temperature and longer time on the LEDs. However, there might be negative effect on the reliability of the PPS-LED by the higher experimental conditions. On the other hand, we applied the new stealth dicing process to form the clean and non-destructive sidewalls. The three times stealth dicing process can get a better well-distributed and shipshape sidewall than one time stealth dicing. The light output power of the PPS-LED had a 39.6 % enhancement compared to the standard LED owing to the larger light-scattering when measured in LED chip form. But the LED with stealth dicing is more reliable than PPS-LED regardless of the crystal quality of the LED.

Delayed texture for improving the thermal stability of Ag reflectors for high-performance GaN-based light-emitting diodes

The thermal stability of Ag-based reflectors for GaN-based light-emitting diodes (LEDs) was significantly improved by inserting an Mg layer. LEDs fabricated with the Mg-inserted reflector exhibited 22% higher output power (at 20 mA) than LEDs with the Ag-only reflector. The Mg-inserted Ag sample was less textured than the Ag-only sample. Based on scanning electron microscopy and X-ray results, the improved stability is attributed to the delay in the texturing and the modification of the surface energy of Ag films.

Sapphire Substrate Processing for High-Performance GaN-Based Light-Emitting Diodes -Micropatterning of Sapphire Substrates and Its Effect on Light Enhancement in GaN-Based Light-Emitting Diodes-

Sapphire Substrate Processing for High-Performance GaN-Based Light-Emitting Diodes -Micropatterning of Sapphire Substrates and Its Effect on Light Enhancement in GaN-Based Light-Emitting Diodes-

High performance GaN-based LEDs on patterned sapphire substrate with patterned composite SiO_2/Al_2O_3 passivation layers and TiO_2/Al_2O_3 DBR backside reflector

GaN-based light-emitting diodes (LEDs) on patterned sapphire substrate (PSS) with patterned composite SiO(2)/Al(2)O(3) passivation layers and TiO(2)/Al(2)O(3) distributed Bragg reflector (DBR) backside reflector have been proposed and fabricated. Highly passivated Al(2)O(3) layer deposited on indium tin oxide (ITO) layer with excellent uniformity and quality has been achieved with atomic layer deposition (ALD) technology. With a 60 mA current injection, an enhancement of 21.6%, 59.7%, and 63.4% in the light output power (LOP) at 460 nm wavelength was realized for the LED with the patterned composite SiO(2)/Al(2)O(3) passivation layers, the LED with the patterned composite SiO(2)/Al(2)O(3) passivation layers and Ag mirror + 3-pair TiO(2)/SiO(2) DBR backside reflector, and the LED with the patterned composite SiO(2)/Al(2)O(3) passivation layer and Ag mirror + 3-pair ALD-grown TiO(2)/Al(2)O(3) DBR backside reflector as compared with the conventional LED only with a single SiO(2) passivation layer, respectively.